'53 Flash and later with the boxed in rear number plate mount have a mudguard profile with a centre raised rib. As mentioned, the wire runs into the underside of the guard adjacent to the regulator mount, then to the rear light supported in the raised rib by the usually rust prone cross tags.
The front edge of the number plate mounting is relieved slightly to accommodate the rib... the dimple mentioned by Wozza on the open type mount is more likely the equivalent for an earlier ribbed guard, rather than a hole to pass the wire from top of guard to lamp.
This layout effectively hides the wiring, but exposes it to the hazards of the detritus found under mudguards.
Swarfy.